Interactions amongst species in a guild of subtidal benthic herbivores

Summary The subtidal coralline flats of northeastern New Zealand support a characteristic guild of grazing herbivores. The most important members of this guild are an echinometrid echinoid, patellid, turbinid and trochid gastropods. Densities of these herbivores fluctuate through time. Interactions within and among the different species of echinoids and gastropods were investigated experimentally. Different combinations of species were caged at densities up to 5 times that of ambient for a 24 week period in an experiment designed to differentiate between intra- and interspecific competition.The echinoidEvechinus chloroticus and the turbinid gastropodCookia sulcata exhibited reduced mean dry weight with increasing intraspecific densities. There was little evidence of density-related mortality in these species. The limpetCellana stellifera showed comparatively large losses of weight and enhanced mortalities in intraspecific experimental treatments but this was not related to density.Investigation of interspecific interactions amongstEvechinus andCookia revealed no evidence of a negative influence of one species on the other. In terms of dry weight,Cookia was indifferent to the presence ofEvechinus, andEvechinus benefited in the reciprocal interaction.Cookia also enjoyed an enhanced mean dry weight when in the presence ofCellana compared to the equivalent intraspecific treatments. There were no coherent trends in proportional mortality in any treatments with enhanced interspecific densities. Cellana, in the presence ofCookia, exhibited a dramatic decrease in mortality rate and increase in mean dry weight. The presence of the turbinid gastropod was clearly beneficial to the limpet when compared to the intraspecific treatments with enhanced intraspecific densities and the control cages containingCellana at ambient density. We suggest that subtidal areas constitute poor habitats for limpets in the absence of agents such asCookia which may provide or maintain suitable sites for attachment and grazing.For the combinations of densities and species investigated there was a consistent trend towards positive interspecific interactions. It seems unlikely that at the sites investigated interspecific competition could act to restrict distributions, or limit abundances of species.     

Detecting non-neutral heterogeneity across a region of DNA sequence in the ratio of polymorphism to divergence

Natural selection, in the form of balancing selection or selective sweeps, can result in a decoupling of the amounts of molecular polymorphism and divergence. Thus natural selection can cause some areas of DNA sequence to have greater silent polymorphism, relative to divergence between species, than other areas. It would be useful to have a statistical test for heterogeneity in the polymorphism to divergence ratio across a region of DNA sequence, one that could identify heterogeneity greater than that expected from the neutral processes of mutation, drift, and recombination. The only currently available test requires that a region be arbitrarily divided into sections that are compared with each other, and the subjectivity of this division could be problematic. Here a test is proposed in which runs of polymorphic and fixed sites are counted, where a "run" is a set of one or more sites of one type preceded and followed by the other type. The number of runs is smaller than otherwise expected if polymorphisms are clumped together. By simulating neutral evolution and comparing the observed number of runs to the simulations, a statistical test is possible which does not require any a priori decisions about subdivision.      

Synchronous biological feedbacks in parrotfishes associated with pantropical coral bleaching

Biological feedbacks generated through patterns of disturbance are vital for sustaining ecosystem states. Recent ocean warming and thermal anomalies have caused pantropical episodes of coral bleaching, which has led to widespread coral mortality and a range of subsequent effects on coral reef communities. Although the response of many reef‐associated fishes to major disturbance events on coral reefs is negative (e.g., reduced abundance and condition), parrotfishes show strong feedbacks after disturbance to living reef structure manifesting as increases in abundance. However, the mechanisms underlying this response are poorly understood. Using biochronological reconstructions of annual otolith (ear stone) growth from two ocean basins, we tested whether parrotfish growth was enhanced following bleaching‐related coral mortality, thus providing an organismal mechanism for demographic changes in populations. Both major feeding guilds of parrotfishes (scrapers and excavators) exhibited enhanced growth of individuals after bleaching that was decoupled from expected thermal performance, a pattern that was not evident in other reef fish taxa from the same environment. These results provide evidence for a more nuanced ecological feedback system—one where disturbance plays a key role in mediating parrotfish–benthos interactions. By influencing the biology of assemblages, disturbance can thereby stimulate change in parrotfish grazing intensity and ultimately reef geomorphology over time. This feedback cycle operated historically at within‐reef scales; however, our results demonstrate that the scale, magnitude, and severity of recent thermal events are entraining the biological responses of disparate communities to respond in synchrony. This may fundamentally alter feedbacks in the relationships between parrotfishes and reef systems.     

Population structure of Placostylus hongii (Gastropoda: Paryphantidae) on the Poor Knights Islands

Abstract

Populations of the native land snail Placostylus hongii (Lesson, 1830) on the Poor Knights Islands were investigated during visits in 1976, 1977, and 1978. Marking and recapture of individual snails showed that linear growth of the shell ceased when a lip was formed around the outer margin of the aperture. Measurements of snails suggested that increase in outer lip width was a good indication of age, but that the rate of lip thickening varied between populations. Large Placostylus showed a restricted range of movement. Both tagging studies and random sampling revealed that mortality was size specific. Most small snails (<60 mm shell length) located were dead, whereas most larger snails were alive. Population densities of living snails averaged 2.2 individuals per 0.25 m² in the study area. The local distribution was patchy owing to the tendency of snails to be associated with specific features of the vegetation.     

The Influence of Antimicrobial Peptides and Mucolytics on the Integrity of Biofilms Consisting of Bacteria and Yeasts as Affecting Voice Prosthetic Air Flow Resistances

The integrity of biofilms on voice prostheses used to rehabilitate speech in laryngectomized patients causes unwanted increases in airflow resistance, impeding speech. Biofilm integrity is ensured by extracellular polymeric substances (EPS). This study aimed to determine whether synthetic salivary peptides or mucolytics, including N-acetylcysteine and ascorbic acid, influence the integrity of voice prosthetic biofilms. Biofilms were grown on voice prostheses in an artificial throat model and exposed to synthetic salivary peptides, mucolytics and two different antiseptics (chlorhexidine and Triclosan). Synthetic salivary peptides did not reduce the air flow resistance of voice prostheses after biofilm formation. Although both chlorhexidine and Triclosan reduced microbial numbers on the prostheses, only the Triclosan-containing positive control reduced the air flow resistance. Unlike ascorbic acid, the mucolytic N-acetylcysteine removed most EPS from the biofilms and induced a decrease in air flow resistance.